1. Field of the Invention
This invention relates to methods for producing biuret, and in particular, it relates to methods for recovering purified biuret from mixtures of biuret and higher molecular weight urea condensation products.
2. Description of the Art
Biuret is widely used in commerce as a precursor for pharmaceuticals, herbicides, and other compounds, as an analytical reagent, and as a ruminant feed supplement. All of these utilities benefit from (if not require) the use of relatively pure biuret.
While biuret can be produced by several chemical methods, it is typically obtained by pyrolyzing urea at a temperature of at least 130.degree. C. and for a period of time sufficient to convert at least a portion of the urea to biuret. An illustrative urea pyrolysis process is discussed by Shipley and Watchorn in British Pat. No. 1,156,099. Unfortunately, as disclosed by Shipley et al., a portion of the urea and biuret are often converted, during pyrolysis, to higher molecular weight urea condensation products such as triuret, cyanuric acid, ammelide, melamine, ammonium cyanurate, methylene diurea, and/or other compounds. Furthermore, a large part of the urea that is manufactured as solid prills is treated at temperatures, during the manufacturing process, that result in some conversion of urea to biuret and higher molecular weight compounds. While the biuret concentration in prilled ureas is typically low, e.g., 0.5 to 3 weight percent, the amount of biuret contained in such products is substantial due to the large volume of prilled urea manufactured annually. The commercial biuret-containing prilled ureas often also contain higher molecular weight urea condensation products such as those mentioned above.
Many of the higher molecular weight condensation products appear to form by the reaction of urea with itself or with previously formed condensation products, or by reactions of, or between, previously formed condensation products. Others, such as methylene diurea, appear to form by the reaction of urea and/or condensation products with additives or other impurities such as formaldehyde which is sometimes employed as a urea anti-caking agent. Regardless of their origin, one or more of such impurities are known to exist in biuret obtained from urea by presently available methods as discussed by Shipley et al., supra, and Kassenbrood in U.S. Pat. No. 3,185,731.
While urea pyrolysis and prilled urea manufacture afford an ample supply of biuret, the major utilities for biuret benefit from the use of that compound in relatively pure form. For instance, analytical procedures and pharmaceutical and herbicide manufacturing practices involving the use of biuret are most often unacceptably complicated by the presence of higher molecular weight condensation products, and the biuret dosage rate which can be employed in ruminant feed supplements is often limited by the toxicity of such impurities.
Methods presently employed to recover pure biuret from mixtures of urea, biuret and higher molecular weight condensation products involve expensive, time consuming, repeated low temperature recrystallization from aqueous solution. The expense involved in such methods obviously increases the cost of pure biuret derived from such sources and limits its application. For instance, ruminant feed supplement manufacturers generally choose to use relatively impure, less expensive biuret at dosage rates which are sufficiently low to avoid the toxic effects of impurities.
Several authors have disclosed that biuret can be removed from urea by contact with the hydroxide ion form of an anion exchanger. For instance Fuentes et al., U.S. Pat. No. 3,903,158 and Takahashi et al., "Determination of Biuret in Urea by Ion Exchange Resins," Soil and Plant Food, Vol. 3, No. 3, Jan. 1958, pages 142-144, disclose that biuret can be removed from aqueous solutions by ion exchange. Neither Fuentes et al. nor Takahashi et al. mention the presence of any other impurities or the possibility that impurity-free biuret can be recovered from mixtures containing higher molecular weight urea condensation products. In fact, Takahashi et al. disclose that "usually, urea for agriculture does not contain nitrogen compounds other than biuret." (Ibid., page 144). While that is often the case, some ureas, in particular those formed by pyrolyzing urea at temperatures above 130.degree. C. for any significant period of time, contain a significant proportion of urea condensation products of higher molecular weight than biuret, some of which are toxic, and all of which can impair product utility.
The use of strongly basic anion exchangers to remove biuret from urea as disclosed by Fuentes et al. and Takahashi et al., supra, suffers from several further disadvantages. Strongly basic anion exchangers such as Amberlite IRA-400 cost in the range of about $50 to about $150 per cubic foot. The strongly caustic or acidic solutions used to regenerate the exchangers are also relatively expensive. Since, according to the literature, the biuret is relatively strongly held by the anion exchanger (a feature which would be beneficial from the standpoint of assuring adequate removal of biuret from the urea solution), the art suggests that relatively severe regeneration conditions are required to efficiently remove the biuret from the deactivated anion exchanger. Obviously, the cost of anion exchanger regeneration, the cost of constructing, maintaining and operating a system capable of removing biuret from a certain quantity of urea solution, and the expense of the anion exchanger required in the process, all increase as the frequency and/or severity of regeneration increases. Thus, the requirement for frequent and/or more severe regeneration increases regenerant costs and the amount of anion exchanger and the size of the operating facility required to treat a given amount of urea solution. The strong base ion exchangers and caustic regenerants used by Fuentes et al. and Takahashi et al. both decompose biuret, and the composition of base by regeneration and by reaction with biuret further increases operating cost.